1. Field of the Invention
The present invention relates to chain extended polyisocyanates and their use as adhesives in making composite materials such as fiber, particle, wafer and strand boards, and plywood. More specifically, the present invention relates to urea extended polyisocyanates.
2. Brief Description of the Prior Art
Composite materials are made by spraying comminuted lignocellulose materials such as fibers, particles, wafers or strands with a binder composition while the comminuted materials are blended by tumbling or agitating them in a blender or like apparatus. After blending sufficiently to form a uniform mixture, the materials are formed into a loose mat. Plywood production is accomplished by roll coating, curtain coating or spraying a binder composition onto veneer surfaces. A plurality of veneers are then laid-up to form sheets of required thickness. The mats or sheets are then placed in a heated press and compressed to effect consolidation and curing of the materials into a board.
The use of polyisocyanates as binders and/or impregnating agents for lignocellulose materials has been proposed with a view to increasing the moisture stability of the materials as well as their mechanical strength. In addition to their quality improving properties, polyisocyanates have far reaching process technical advantages when used as binders, as have been disclosed in German Offenlegungsschrift No. 2,109,686.
U.S. Pat. No. 3,870,665 discloses the use of polyisocyanates and catalysts therefor in the manufacture of plywood, fiberboard, and other compression molded articles.
The large scale industrial manufacture of composite materials which are bonded exclusively with polyisocyanates have previously been limited. The use of some of the polyisocyanates, particularly the better performing isocyanates, such as polymethylene diisocyanate has been limited by their cost. Because of the cost constraints, the level of use of these expensive isocyanates is kept low for a given material. One approach to the use of levels of these isocyanates has involved chain extending the isocyanates with inexpensive extenders.
By the present invention, there is provided an inexpensive but effective urea extended polyisocyanate.